Rapid thermal decomposition (pyrolysis) in the absence of oxygen is a process to extract hydrocarbon liquid from woody bio-mass as a potential petroleum substitute. Pyrolysis oil, also known as bio-oil, has properties such as low heating value, incomplete volatility, acidity, instability, and incompatibility with standard petroleum fuels that significantly restrict its application. The undesirable properties of pyrolysis oil result from the chemical composition of bio-oil that mostly consists of different classes of oxygenated organic compounds.
The elimination of oxygen is thus necessary to transform bio-oil into a liquid fuel that would be accepted as transportation fuel and economically attractive. Two types of processes are generally used to remove oxygen from organic molecules: catalytic cracking and hydrotreating.
Catalytic cracking removes oxygen in the form of water and carbon oxides using shape-selective catalysts. Catalytic cracking accomplishes deoxygenation through simultaneous dehydration, decarboxylation, and decarbonylation reactions occurring in the presence of catalysts. In the past, zeolite such as ZSM5 catalysts has been used to perform cracking. Other catalysts such as molecular sieves (SAPOs), mordenite and HY-zeolite have also been utilized. The extent of coking (8-25%), high extent of formation of light ends (gas-phase hydrocarbons) and low quality of final fuel grade products are prohibitive towards a scalable cracking process. All these factors result in carbon and hydrogen loss thereby reducing both carbon and hydrogen efficiencies.
Hydrodeoxygenation (“HDO”) is considered the leading technology to achieve oxygen removal from bio-oil. HDO also known as hydrotreating involves high-temperature, high-pressure processing in the presence of hydrogen and catalyst to remove oxygen in the form of water. HDO consists of contacting bio-oil with hydrogen at high pressure and high temperature in presence of a catalyst. Both of these processes require new equipment wherein the capital expenditure is significantly higher. Moreover, the catalyst is susceptible to sulfur and phosphorus impurities in bio-mass. Most of the catalysts used for hydrodeoxygenation are some variations of Co—Mo or Ni—Mo impregnated on a support. Many investigators have focused upon alumina as a preferred catalyst support. Others have investigated carbon, silica and zeolite based supports.
However, HDO suffers from significant challenges, including: 1) coking, which limits the catalyst lifetime; 2) polymerization of various compounds in bio-oil before deoxygenation due to sequential nature of bio-oil productions and catalytic treatment; 3) deactivation of HDO catalysts by the presence of water in the pyrolysis oil (deactivation occurs by leaching sulfur from active sites since these catalysts are usually sulfided prior to HDO process to alleviate coking); 4) hydrothermally unstable nature of zeolite based catalysts compared to noble metal catalysts, which are cost prohibitive; 5) requirement of significant quantities of hydrogen to remove oxygen (cost of hydrogen is approximately $1.50 per gallon of product hydrocarbon); 6) economic availability of hydrogen at distributed smaller scale suitable for bio-mass conversion; and 7) significant process exotherm due to high oxygen removal requirement (25% by mass), which consequentially requires high recycle rates at commercial scale to manage the heat, thereby contributing to high processing costs.
Thus, there are numerous challenges that prevent commercialization of bio-oil upgrading to hydrocarbons process. An alternative economically feasible, hydrogen independent and decentralized process is needed to convert bio-mass derived pyrolysis oil to refinery ready hydrocarbons with an increased energy density.